Yellow gold above 14k is also hypoallergenic, depending on how sensitive her allergy is--24k may work. However, if she wants the silver color, you'll probably have to go with platinum, even if it does cost a fortune. White gold can have nickle in it, so that won't work. And if you give her a sterling silver ring (which *is* hypoallergenic), she may not like that.
I don't know if there are any other benefits, other than it's really valuable and pretty rare. That could also be a draw-back if you are on a budget. Platinum is harder than gold, so it's more durable, but it still can scratch. Some people say it scratches easily. It's also pretty heavy.
You may also look into titanium, which is a bit harder to find in jewelry stores, but it is hypoallergenic (so much so that they use it in surgical implants) and it's much cheaper. It's also very long-wearing, light, and durable. It might be something to look into.
Congratulations on your engagement! Regardless of what it's made of, I'm sure she'll love the ring because she loves the man who gave it to her.